Integrated circuits, such as microprocessors and memory devices, contain many MOSFET transistors on each device, providing the basic switching functions required to implement logic gates and/or data storage. In one example, a DC-to-DC power converter, such as a buck converter, usually employs MOSFETs as its switching devices. In particular, a buck converter widely used throughout the industry converts a higher input voltage into a lower output voltage. A synchronous buck converter includes a pair of switching devices coupled in series across the input voltage source. The switching devices are typically MOSFETs. FIG. 1 shows a conventional buck converter which consists a high-side and a low-side switch. The high-side switch 102 is coupled to the voltage supply (Vin) and the low-side switch 104 is connected to ground. An output filter typically including an inductor (L) and a capacitor (Cout) is connected to a junction (i.e., phase node or switch node) 105 formed by the pair of switches 102 and 104 for providing the output voltage to a load. A controller 110 drives the switches to connect the output filter to the voltage supply or to ground to maintain the output voltage at a predetermined level.
One issue in a synchronous buck converter design is voltage overshoots and ringing on the phase node. Due to the growing demand for higher performance devices, the high side MOSFET's switching speed are expected to be faster. The faster the switching speed, the faster the voltage transition of the phase node. The fast voltage transition may cause the inadvertent turn-on of the low-side switch, resulting in a converter malfunction and reducing efficient. In addition, it may cause the phase node ringing as the magnitude of the ringing is a function of the high side MOSFET's switching speed. The phase node voltage spikes can induce malfunction or undesired stress of the power switches.
A number of techniques are used to control phase node ringing. As shown in FIG. 1, the boost resistor RBOOT and the high side gate resistor RGH have been included in the synchronous buck converter of FIG. 1 for control of the phase node ringing phenomenon. Specifically, the boost resistor RBOOT can slow down the turn-on of the high side MOSFET without affecting the turn-off. In addition, the high side gate resistor RGH in series with the gate can increase both the turn-on and turn-off times of the high-side MOSFET, which controls ringing on the rise and fall of the phase node. Another proposal is to integrate a Schottky diode in the low-side switch to reduce recovery charge, thus avoiding voltage spikes at phase node. It is desirable to design a switch circuit that can properly control the phase node ringing of a synchronous buck converter.
It is within this context that embodiments of the present invention arise.